1. Field of the Invention
The present invention relates to a method of generating ozone by electrolysis, an ozone generating system employing an electrolytic method and an ozone water producing system. Furthermore, in these systems or methods, the present invention relates to improvements in water utilization technologies and water modification technologies wherein the water is consumed by those electrolytic reaction or is used for other purposes.
2. Description of the Related Arts
Ozone (O3) has strong oxidizing and sterilizing power and is used for deodorization and sterilization. Artificial ozone generating methods are classified into two types of methods, i.e., in-gas electric discharge methods and in-cell electrolytic methods. The former xe2x80x9cdischargexe2x80x9d ozone generating methods make high-voltage electricity discharged in air or in a pure oxygen atmosphere to produce ozone from a certain part of the supplied oxygen. Belonging to the latter methods, most electrolytic ozone generating methods use an electrolysis cell that employs a hydrogen ion exchange membrane (PEM, or proton exchange membrane) as a solid electrolyte for electrolysis of water. The electrolysis cell is formed by disposing, on one side of the PEM, an anode with an ozone generating catalyst, such as lead dioxide on a collecting member and, on the other side of the PEM, a hydrogen generating electrode (cathode) having platinum or the like material. Ultrapure water is supplied at 0.1 (S/cm to the anodic side in the cell, and electric current of a current density in the range of 0.5 to 2 A/cm2 is passed through the electrodes. Consequently, a gas containing oxygen (O2) and ozone (O3) in an oxygen-to-ozone ratio of about 9:1 is produced on the side of the anode.
The xe2x80x9cdischargexe2x80x9d ozone generating method tends to involve problems due to generation of electromagnetic noise and production of annoying nitrogen oxides. Although any nitrogen oxides will not be produced if pure oxygen is used, cylinders for storing pure oxygen and a large-scale piping system would be required in order to utilize pure oxygen. Such large-scale facilities cannot be installed in many places. Still further, generation of electromagnetic noise is unavoidable in any of such xe2x80x9cdischargexe2x80x9d type methods.
Those typical problems in the xe2x80x9cdischargexe2x80x9d type methods are not involved in the xe2x80x9celectrolyticxe2x80x9d type methods. Further, location of facilities for the electrolytic ozone generating method is flexible because its primary requirements, i.e., the water supply and power supply, are easily available in many places. The electrolytic method is suitable for use in a place where many electronic devices that must be shielded from electromagnetic noise are located and where very clean sanitary environment is also required. Typical examples of such places are medical facilities including hospitals, clinics, medical laboratories, etc.
Ultrapure water has been used for the above-mentioned electrolytic ozone generation methods in recent years. If not, in other words, if many electrolytic impurities are contained in the water, the water has a high ion conductivity. Thus, electric current expected to flow from the collector through the ozone generation catalyst (i.e., lead dioxide) to the PEM skips the catalyst, and flows directly to the PEM. Consequently, ozone generating efficiency becomes poor. Further, if the water contains metal ions, such as calcium ions, the metal ions tend to combine with sulfonate groups contained in the PEM, thereby tending to deteriorate the proton conductivity of the PEM and to increase the electric current density and resistance. As a result, the electrolysis cell generates heat and, at worst, it is possible that the electrolysis cell is broken. To avoid such a effect, the electrolytic ozone generator requires a water purifying system to obtain ultrapure water.
A conventional water purifying system to produce such ultrapure water accepts service water as raw water. The system has an ion-exchange filter device connected to the service water supply passage. The ion-exchange filter device has a container packed with ion-exchange resin beads, i.e., resin beads for fixedly holding positive ions and negative ions on their surfaces. When the service water or the like water is passed through the container, electrolytes contained in the passing water are replaced with water molecules.
Generally, service water has an electrical conductivity in the range of 150 to 300 xcexcS/cm. When purifying such raw water to produce ultrapure water of 0.1 xcexcS/cm in electrical conductivity, the ability of the ion-exchange resin will be reduced in a short time and the ion-exchange resin must be changed after short periods. Generally, in the case of purifying service water of 150 xcexcS/cm to produce ultrapure water of 0.1 xcexcS/cm, the amount of raw water that can be processed through a life span of the ion-exchange resin is about 200 times the volume of the ion-exchange resin. For example, when ultrapure water is produced continuously at 100 mL/h by using 1 L of ion-exchange resin, the ion-exchange resin must be changed at intervals of about 1.5 months. Although the ion-exchange resin changing period can be extended by increasing the amount of the ion-exchange resin, a large container is necessary to contain a large amount of ion-exchange resin. As a result, the size of an ozone generator including this water purifying system increases accordingly, which involves the reduction of space efficiency and the increase of transportation cost. Needless to say, a housing for containing the large volume of ion-exchange resin will be expensive.
The conventional water purifying system has many problems in practically applying generated ozone gas to sterilization and disinfection. Generally, when ozone is used for sterilization in medical facilities, it is usual to use an ozone water producing apparatus having a mixing system connected to an ozone gas producing system to dissolve ozone gas into water. The resultant ozone water is used as cleaning medium for washing hands and the like.
Ozone water must be accurately adjusted in a proper ozone concentration so that the ozone water has effective sterilizing power and is still harmless to the human body at the time of its use. A final ozone concentration adjustment can be achieved by a diluting step. However, the final ozone concentration adjustment tends to be difficult if the ozone concentration of undiluted ozone water is unstable. Therefore, if we focus on the ozone concentration of the ozone water at the time of just being delivered from the mixing system, it is desirable that the concentration is strictly controlled. However, since ozone is a very unstable substance, it reacts readily with organic substances contained in water and decomposes into oxygen. Particularly, it is known water contains organic substances that react sensitively with ozone. We call them xe2x80x9cozone-consuming substancesxe2x80x9d. If the raw water contains the ozone-consuming substances in a large concentration, consumption or decomposition of ozone dissolved in water is accelerated. As a result, the time for which ozone is able to stay in water is reduced. Besides, the half-life of ozone dissolved in water is shorter than that of ozone existing in gas. Therefore, in recent years, it is general to use ultrapure water as water for dissolving ozone. To suppress the consumption and decomposition of ozone as effectively as possible, it is desirable that ultrapure water has a very small organic substance concentration, not to speak of a very small electrolyte concentration. Generally, ultrapure water is required to have a conductivity in the range of about 0.01 to about 1 xcexcS/cm and a total organic concentration (TOC) in the range of about 10 to about 50 ppb.
However, the above mentioned ion-exchange resin is unable to remove nonelectrolytes. Therefore, even using such ion-exchange resin, it still tends to be difficult to control the ozone concentration of ozone water. This difficulty depends on quality of service water (or the like raw water) which is converted into the conventional ultrapure water. Sometimes, acids produced by the interaction of ozone and organic substances deteriorates the PEM and electrodes. Therefore, conventional ozone water producing system has been requiring an expensive, large-scale water purifying system in order to reliably remove organic substances from water. This has been a factor that made miniaturization and cost reduction of the ozone water producing system very difficult.
Accordingly, it is a first object of the present invention to provide a novel electrolytic ozone gas generating method that enables the miniaturization of an ozone generating apparatus, the reduction of manufacturing cost and the reduction of running cost through the improvement of a water purifying system employed therein.
A second object of the present invention is to provide an ozone water producing system including at least an improved portion other than the ozone gas generating portion, wherein the improved portion contributes to suppression of consumption and decomposition of ozone in ozone water in ozone water storage part. This is done through an effective utilization of an electrolytic ozone generator including the improved water purifying system, and an effective utilization of raw water purified by the water purifying system (hereinafter referred to as xe2x80x9cozone-gas-generating raw waterxe2x80x9d, the entirety of which is novel.
The inventors of the present invention developed a water purifying process including a raw water processing process using a reverse osmosis membrane (RO membrane) as a method capable of extending an ion-exchange resin maintenance cycle for the maintenance of ion-exchange resin for producing ultrapure water necessary for electrolytic ozone generation and of producing satisfactorily purified ultrapure water. The inventors of the present invention found a further fact. Water practically acceptable in capability to suppress the consumption and decomposition of ozone mixed and dissolved therein was obtained during the inventors"" water purifying process, even if the general-purpose purity evaluation of the water after the purification is not as high as that of conventional ultrapure water. Such water will be called xe2x80x9clow-ozone-consuming pure waterxe2x80x9d.
The present invention has been-made on the basis of the foregoing techniques and knowledge.
A first aspect of the present invention is characterized by the following terms (1) to (5).
(1) It is a method for generating ozone comprising an electrolytic process.
(2) RO-membrane-processed pure water is produced by processing service water by a reverse osmosis membrane water purifier.
(3) Low-ozone-consuming pure water is obtained by removing as much xe2x80x9cozone-consuming substancesxe2x80x9d as possible from the RO-membrane-processed pure water. Such removal is achieved by specific-substance-removing means.
(4) Electrolysis raw water having an electrical conductivity in the range of 0.01 to 2.00 xcexcS/cm is obtained by reducing electrolytes contained in the low-ozone-consuming pure water by processing the low-ozone-consuming pure water by ion-exchange resin.
(5) The electrolysis raw water is introduced into an electrolysis cell device and is subjected to electrolysis. Ozone-containing gas generated from the anodic side of the electrolysis cell is separated from the electrolysis raw water.
The specific-substance-removing means may be active carbon fibers.
According to a second aspect of the present invention, an electrolytic ozone generating system comprises a reverse osmosis membrane water purifier for processing service water to produce pure water; a specific substance-removing device that receives the pure water from the reverse osmosis membrane water purifier and removes ozone-consuming substances from the pure water to produce low-ozone-consuming pure water; an ion-exchange treatment device that receives the low-ozone-consuming pure water from the specific-substance-removing device and removes electrolytes from the low-ozone-consuming pure water in order to obtain electrolysis raw water having an electrical conductivity in the range of 0.01 to 2.00 xcexcS/cm; an electrolysis cell device that receives the electrolysis raw water from the ion-exchange treatment device and electrolyzes the electrolysis raw water; and a gas-liquid separating vessel that separates ozone-containing gas generated from the anodic side of the electrolysis cell device from the electrolysis raw water and collects the ozone-containing gas.
In the electrolytic ozone generating system according to the second aspect of the present invention, the specific substance removing device may be a filtering device employing active carbon fibers.
The foregoing electrolytic ozone generating system may use condensed drain discharged from the reverse osmosis membrane water purifier as a heat-exchanging means for cooling the electrolysis cell device and the gas-liquid separating vessel.
According to a third aspect of the present invention, an ozone water producing system comprises, in addition to the components of the foregoing electrolytic ozone generating system, a closed gas-liquid mixing vessel, a water supply control mechanism for supplying the low-ozone-consuming pure water discharged from the specific-substance wise-removing device into the gas-liquid mixing vessel to maintain water level within suitable range so that both a gas phase and a liquid phase exist in the gas-liquid mixing vessel, a temperature control mechanism for maintaining water contained in the gas-liquid mixing vessel at a proper temperature, a ventilating mechanism for continuously introducing the collected ozone-containing gas collected by the gas-liquid separating device into the gas-liquid mixing vessel and for discharging the gas so that the pressure in the gas-liquid mixing vessel may not vary extremely, and a mixing means for continuously stirring and mixing the liquid and the gas contained in the gas-liquid mixing vessel, wherein ozone gas is dissolved in the low-ozone-consuming pure water supplied into the gas-liquid mixing vessel to obtain ozone water.
The foregoing ozone water producing system according to the third aspect of the present invention may further include an ozone water discharging mechanism for discharging the ozone water from the gas-liquid mixing vessel at a predetermined flow rate, and an ozone water diluting mechanism which supplies simply filtered service water to the ozone water on its way to be discharged by the ozone water discharging mechanism wherein such supply of the filtered water is done at a predetermined flow rate to dilute the ozone water.
In the foregoing ozone water producing system according to the third aspect of the present invention, the temperature control mechanism connected with the gas-liquid mixing vessel may use condensed drain discharged from the reverse osmosis membrane water purifying device as a heat exchanging means.